Method of blowing sand into sand molds



Jan. 29, 1957 H. J. B. HERBRUGGEN METHOD OF BLOWING SAND INTO SAND MOLDSOriginal Filed May 8. 1952 2 Sheets-Sheet 1 Jan 29, 1957 H. J. B.HERBRUGGEN METHOD oF BLowING SAND INT0 SAND MoLDs Original Filed May 8.1952 2 Sheets-Sheet 2 Y L 4/ f1 MNR United States Patent() NIETHOD FBLOWING SAND INTO SAND MOLDS Heinrich J. B. Herbruggen, Cleveland, Ohio,assigner yto The Federal Foundry Supply Co., Cleveland, 0h10, acorporation of Ohio L Original application May 8, 1952, Serial No.286,774. Divided and this application March 22, 1954,.Serial No. 417,805

5 Claims. (Cl. 22-193) This invention relates to a control system for acore or mold blowing apparatus and more particularly to a system andapparatus for automatically determining the blowing cycle while limitingthe pressure in the mold 0r core box and the rate of positioning themold or core box into blowing position. This application is a divisionof my application Serial No. 286,774 led May 8, 1952, and entitledControl System for Sand Blowing Apparatus.

A sand blowing apparatus usually embodies some means such as a sandtransfer chamber for delivering sand to a receptacle in the form of amold or core box. delivery of sand from the chamber may be aided by uidpressure or by the combination of fluid pressure and an agitator. Theapparatus may also be provided with means such as a lifting mechanismfor positioning and clamping the sand receptacle and sand transferchamber in sand blowing relation. A desirable feature for such an ap-`paratus relates to the provision of an automatic blowing cycle withproper sequential control of the receptacle positioning and delivery ofsand thereto.

Additional desirable features relate to safety precau tions in theoperation and control of the apparatus. For example, the conventionalcore blowing apparatus utilizes clamping and blowing pressures whichmany types of core boxes are too frail to withstand. Accordingly, whenusing sand receptacles made of wood or the like, it has been necessaryin the past to make the cores by hand or to copiously vent thereceptacle for use with the conventional blowing apparatus. Copiousventing, however, has inherent disadvantages in the construction of thereceptacle as well as providing unsatisfactory and inferior cores ormolds. Furthermore, utilization of uncontrolled uid pressure inconventional core blowing apparatus requires cumbersome and uneconomicaldesign of the operative elements. For example, it is common practice inconven-k tional types of apparatus to make the lift or clamping pistonsslightly larger than the largest sand receptacle.

The positioning of the mold or core box into blowing relation with asand transfer chamber presents additional problems in handling duringthe blowing operation, since the usual positioning or lift mechanismutilizes the full clamping force to position the mold or core boxagainst the transfer chamber and causes damage to the apparatus and tothe box.

Accordingly, it is an object of this invention to provide an apparatusand control having an improved automatic blowing cycle with propersequential control of sand receptacle positioning and sand deliverythereto.

Another object of this invention is to controlithe sand deliverypressure during the blowing cycle so as to limit the pressure in thesand receptacle to a predetermined safe value.

Still another object of this invention is to limit the rate of travel ofthe sand receptacle positioning mechanism to a predetermined safe rateuntil the receptacle and sand transfer chamber are in blowing positionand then to automaticallysupply an increased clamping pressure -for theblowing operation.

The Y 2,779,074 Patented Jan. 29, 1957 ICC Still another object of thisinvention is the reduction in size of the sand receptacle lift mechanismto provide a lighter and more economical apparatus.

VA further object of this invention is to automatically terminate theblowing'cycle when the sand receptacle is filled with sand.

A still further object of vthis invention is to automatically initiatethe blowing operation when the sand receptacle is properly positioned inblowing relation with the sand transfer chamber.

Briey, in accordance with this invention there is provided a closedrigid frame having a sand transfer chamber within the top portion andsurmounting a sand receptacle lift mechanism vwithin the lower portionof the closed frame. The transfer chamber is provided with a controlledfluid pressure inlet and the apparatus includes a device to regulate thefluid pressure admitted to the chamber inlet. The apparatus alsoincludes a fluid pressure device that operates in response to apredetermined fluid pressure in the chamber consequent upon thereceptacle being lled with sand to remove the fluid pressure from thechamber and thereby automatically terminate the blowing cycle.

A sand receptacle positioning mechanism is providedf which embodies alift piston that is initially actuated by a reduced uid pressure tolimit the rate of lifting travel to a predetermined saferate. tion iscontrolled by iluid pressure devices which respond to a developedpressure within the lift piston'cylinder'when the piston has reached thelimit of its travel tov supply an increased fluidv clamping pressure tothe lift piston during the blowing cycle.

Further in accordance with this invention, the controlled fluid pressureinlet for the sand delivery mechanism may be-vcoupled to the liftingmechanism so that it will respond to the developed pressure in the liftpiston cylinder incident to the positioning of the sand receptacle inblowing relation to initiate the blowing cycle and the entire system maybe coupled together to automatically initiate sand delivery and safetycontrols in a predetermined blowing cycle.

In the drawings, f

Fig. 1 is a partial vertical section of the blowing apparatusschematically coupled together with the mechanical controls of thesystem;

Fig. 2 is a schematic circuit diagram for electro-magneticallycontrolling the blowing cycle while maintaining safety precautionsduring the blowing operation;

Fig. 3 is an enlarged partial sectional view of the device whichregulates the pressure in the sand receptacle;

Fig. 4 is a schematic circuit diagram of an electromagnetic controlsystem for regulating the pressure in the sand receptacle;

Fig. 5 is an enlarged view, partially in section, of the fluid pressuredevice which automatically determines the blowing cycle;

Fig. 6 is a schematic circuit diagram for'electro-magneticallycontrolling the pressure in the sand receptacle and terminating theblowing cycle;

Fig. 7 is a partial sectional view of the sand transfer chamber exhaustin a modified form; and

Fig. 8 is a partial view, in section, of the blow and vent-plateassembly at the discharge opening.

Referring to Fig. 1 of the drawings, there is shown a. sand blowingapparatus embodying a closed frame havingv The clamping ac A sandtransfer chamber 14 is operatively supported within the upper portionofthe frame by arms 15 and 16 which are pivotally connected to a journal17? 3 rotatably. mounted@ diagramma. 1.2,-` A sandhopper 1,8 issupported externally of the closed frame on the column l2 and above theplane of the sand transfer chamber 14. The hopper is providedwithadischarge opening 19 which is normally closed by a pivot gate andsurroundedby.

a loose scraper ring 21. The arrangement is such that the sand transferchamber 14 can berotated onits'journal 17 about the column 12, intolling position under the hopper 18 and then can be swung back within theclosedframe into core blowing position.

Within the lower portion of the closed frame, there is provided acylinder 2 2 which supports a movable lift piston 23 for verticalmovement towardsand awa-y from the sandtransfer chamber.v Thelift-piston 23 supports a clamping table 24 on which a sandjreceptacle`.such yas a mold or .core .box 25 Vis lplaced forpositioning andclampingfinflblowingrelation against thesand transfer chamber. Thetermycore boxwillhereinafterbe used to describe either l.a core `box or moldforconvenience vof description.

Thesandtransferchamber 14 forms ablow-head'which is .pivotally supportedatthe .ends-of the arms-15, and 16 for .vertical movement ,parallel tothe columns 12 and 13 within the closed frame. Thus, when avcore box-25is placedon `the `clamping table 24and the .lift piston v23 is actuatedtov liftathecore box-into engagementv with the underside of the sandtransfer chamber, the enti-re asf semblyof `core boxl andtransfercharnber istpositioned against .thecross head 11. The fillingopening 26 of the transferchamber communicates through a filter 27 withanexhaust space 28 in the cross head having an exhaust valve 29-.whichmay be opened or closed -as will be heref inaftermore fully described, y

"The sand transfer chamber 14 hasan inner perforated partition 30formingan inner sand room-surrounded by an air jacket in communication with afluid pressure inlet 33 .at the top of the transfer. chamber. Fluidpressure is admitted through the inlet-A 33 into the air jacket andpasses throughthe perforations in the partition 30 into the sandroom toforce sand through a discharge opening 34 at thebase of the chamber. Thesand room in the transfer;chamber..containsL an agitator 35 which isrotated by ageari ring36 supported at the top of the transfer charn-Vber.v l The gear ring 36 is driven by a spur, gear 37 which is'locatedin the air jacket for protection against sand and dirt. The spur gear 37may be driven by any suitable means such Avas an electric motor. Thebottom of the sand transfer chamber 14 isclosedby a removable blow-plate38 which is coupled to the chamber through a spider by bayonettypelugsn39tfor ready maintenance and interchange. s

The blow `plate L38` preferablyhas a single sanddischarge opening `34inthe kcenter and carries a ventiplate 40 which adapted to engage thetop of the core box 25 fornventing the box to the atmosphere. Othervents .41 around vthe discharge opening 34 of .the blow-plate do notcommunicate with the free air but are -connected through a ventingchamber 42 with a fluid pressure operatedzdevice 43 :as best lshown inFig.I 8 `of the4 drawings. The.

vents are suitably spaced from the discharge opening 34 toavoidtheescape of sand from the core box.

The transfer chamber exhaust valve 29 is spring biased in a normallyopen position and the uid pressure .inlet S35i-'inthe crossnhead 11contains a .spring-biased valve 44 which normally ,isolatesthe chamberinlet 33 'from a sourceoffluid pressures connected to the cross head 11bythe main supplyline 45. Fluid pressureis also led from .thesupplylline 45 to the casings 48 and 49 of normally` closed valves and 51 whichin turn have control condnitshSSjvand 56 leading to the cross head andto the liftpistonLcylinder'respectively. The control conduit 55 leadingfrom the valve casing 49 has a branch conduit 57 communicatinglwitl'ithe underside-58 of the inlet valvev 44 to oppose: the springload'and anotherfbranch conduit thawing.loropathatvalva i.

In the preferred embodiment shown in Fig. 1v, each of the valves 50 and51 are electro-magnetically positioned by means of solenoi'ds 60 and 61acting on the respective piston rods 62 and 63. When the solenoid 60 isenergized it positions the valve 5G. to establish communication be tweenthe conduit 56 leading to the lift piston cylinder 22 andthemain supplyline 45 to positionv the core' box25| in core blowing relation againstthe sand transfer charnberf14'. In like manner, when the solenoid 61 isenergized it positions the valve-51l to establish communication betweenthe main supply line 45 and the branch conduits 57 and 59.v Thearrangement is.` such that fluid pressure acting on the under side 5S ofthe inlet valve 44 opens the inlet conduit 46 to pressure from thesupply -line 45 lwhile Huid pressure acting on the exhaust Valve 29counteracts the spring load and isolates the transfer chamber 14 fromthe exhaust opening 47 in the cross head 11. The energization of each ofthese solenoids 60 and 61 may be hand-controlled by means of a circuitswitchl toV initiate and stop the core blowing ycycle or may beautomatically controlled by a circuit as shown in Fig. 2 to remainenergized fora predetermined core-blowing cycle, as will be hereinaftermore fully described.

Referring now to Fig.` 3 of the drawings, there is shown anfenlargedView of the uid pressure device 43 that communicates with the ventinglchamber 42 which surrounds the discharge opening 34 in the blowplate38. This de# vice has aU-shaped body with pistons and 66 freelysupported in eachy leg for movement'along a common axisparallel to thebase of thebody. Each piston 65 and 66 surmounts a uid pressure path 67and 68 communieating -with the venting chamber 42. When fluid pressureissupplied vfrom the core boxk 25 through the ventingl chamber 42 to theiiuid pressure device, each of thev pistons 65-and -66 are vlifted inthe same direction. A' lever arm 70'is pivotally connected to the baseof the U- shaped body and extends between the legs of the body tolintersect the common'aligned axis of the freely supported pistons 65land 66. The-lever arm 70 is biased by a spring 71 'which is-connectedto an off-center position 72 on the arm 70wso as to retain the arm ineither one of two positions noverlying one `or the other of the pistons65 or 66. The spring load on the spring 71 is selected so thatit will beovercome by the force of piston 65 acting undera predetermined safe`value of fluid pressure in the corebox 25 ltofsnap-the lever arm 70from its lower position-to its upper position between the legs of theU-' shaped body.`

At the free extremity of the lever arm 7 0 is a mercury switcht73 which,in the preferred embodiment shown, is

normally closed in the lower position of `the lever arm so that when-thelever arm is ltilted upwardly the -mercurytravels' by gravity to thelower end of the Iswitch capsule -74 land `opens the circuit between thewires con nected thereto. These wires may be connected inasimplehand-controlled1st-art and stop circuit as shown 'in Fig. -4ofthe drawings, so that when the lever arm 70 is tripped` to open themercury switch 73 it also deenergizes the solenoid 61 to close the valve51 and allow the spring biased inlet valve A44 to isolate the sandtransfery cham-v ber 14 from the main u-id pressure supply line 45. Thepressure is also removed from the spring-loaded exhaust valve-.29 whichopens and allows the transfer chamber 14 to exhaust to the atmosphere,thereby causinga re' duction of fluid pressure` in the chamber andinthe" core box-untilfsuch time as the freely supported pistons 65 and.66'inf'the fluid pressure-device 43 drop back to -theirinitialpositions by virtue of gravity and swing the lever arm 70 back toits initial position to again close'themercury switch .73 and energizethe solenoid 61M This operation-may take vplace a number of times duringa core blowing 'cycle and functions .to regulate lluid. pressureingtheicorefbox ,25 by pulsing the admission of fluid pres sure to the`sand transfer `chamber14 so .that itnever reaches a value greater thanthecoreA box willfwith'stand:

the blowing cycle. Referring again to Fig. 1, the preferred embodimentshown there may be operated to automatically determine the duration ofthe core blowing cycle by virtue of the pressure operated device 75. Anenlarged view of this device is shown in Fig. of the draw ings in theform of a differential fluid pressure operated device carrying a mercuryswitch 76. The differentialA device 75 is in the form of a scale arm 77pivotally supported intermediate its ends which are in the form of valvechambers 73 and 79 open at the base to receive fluid pressure orifices80 and 81 which in turn are connected through suitable conduits 82 and83 to the air jacket of the sand transfer chamber 14. Each 4of theorifices 80 and 81 is provided with a washer 84 and 85 to allowexpansion of the fluid pressure wi-thin the closed end of thecorresponding valve chamber formed in each end of the scale arm 77.Thus, an unbalance of pressure between the orifices 80 and 81, oradifference in the length of the scale lever arms or in the washer sizeswill cause the scale arm 77 to tilt or rotate about its pivotal support.

In the preferred embodiment shown in Fig. 1 of the drawings, one of theorifices 81 is coupled to the air jacket adjacent the inlet end of thetransfer chamber 14 while the other orifice 80 is coupled to the -airjacket in spaced relation adjacent the discharge end of the transferchamber, so that while the core box 25 is being filled with sand and thefluid pressure is flowing from the inlet 33 of the transfer chamberthrough the discharge opening 34 there is a differential of pressurebetween each of the orifices 80 and S1. The scale lever arms are such`that the differential of pressure through the transfer chamber y 14will maintain the scale arm 77 in a position to render the mercuryswitch 76 normally open. As soon as the core box 25 is filled with sandhowever, fluid pressure within the transfer chamber equalizes and thescale arm 77 is tilted Ito closethe mercury switch 76 and energize asolenoid 86 in an electro-magnetic control circui-t to deenergize the4solenoids 60 and 61 and .thereby automatically terminate thecore-blowing cycle as shown in schematic -circuit diagram -of Fig. 6 ofthe drawings.

In the circuit diagram of Fig. 6, kthe core blowing cycle is initiatedby means of a momentary push button 87 which energizes .the solenoid 83to close the normally open contacts 89. The closing of the contacts 89energizes the solenoid 60 and the solenoid 61 through the normallyclosed mercury switch 73. Since the push button 87 is a momentary pushbutton, means are provided for holding the solenoid circuits energizedafter the push button 87 is released. As shown in Fig. 6, this isaccomplished by latching the contact arm 90 with the spring biased roll91 to maintain lthe energizing circuit through the solenoids 60 and 61.This latch will maintain the energizing circuit until the mercury switch76 is closed to energize the latch solenoid 86 which provides anunlatching force greater than the bias of the spring 92 on the latch arm93 to remove the latch and open the contacts 89, thereby deenergizingthe solenoid circuits 60 and 61 until such time as the push button 87 isagain operated. n

The control and regulation of the pressure within the core box 25, whichwas described in connection with Figs. 2 and 3 of the drawings as apulsating operation which periodically closed the fluid pressure inletto the transfer chamber 14 .and opened the chamber exhaust 47, may bemodified so that the chamber exhaust 47 remains closed .and only -thefluid pressure inlet 33 is pulsed so that the The relief of thispressure is accom` ya lesser extent each vtime that the safe limit :offluid presi sure in the core box 25 is attained. This modified form',wherein the exhaust remains closed during the pulsing of the fluidpressure inlet, saves the time of rebuilding the fluid pressure to theblow pressure .and may be advantageous in some circumstances.

A preferred way of modifying the apparatus to enable it to operate inthis modified manner is shown in the partial sectional view of thecrosshead in Fig. 7 of the drawings. In this modified construction, thefluid pressure conduit 59 in the crosshead 11, which counteracts theloading spring 52 to close the exhaust Valve 29, is provided with a oneWay check valve 53 which prevents the removal fof fluid pressure fromthe exhaust valve 29 when the solenoid 61 is deenergized by the pulsingmercury switch 73. The exhaust valve chamber is provided with anatmospheric release vent 54 from the crosshead which is normally closedby means of anl the mercury switch 76 so that when the core blowingcycle` is automatically terminated bythe oper-ation of the switch 76 thesolenoid 94 is also energized and opens the atmospheric vent 54 to theexhaust valve chamber, thereby enabling the exhaust valve 29 to open andexhaust they sand transfer chamber.

As previously noted, energization of the solenoid 60- also establishescommunication between the main fluid pressure supply line 45 and thecylinder 22 of `the lift piston to position the core box in core blowingrelation against the sand transfer chamber. In the preferred ernbodimentshown in Fig. 1, the conduit 56 supplies such fluid pressures through aone way check valve directlyto the underside of the lift piston 23 inthe piston cylinder. The conduit 56, or the inlet from valve casing 50,is of lesser cross sectional area than the main fluid pressure supplyline 45 so as to restrict the flow of fluid pressure to the lift pistoncylinder. This results in a restricted flow of pressure being suppliedto the lift piston cylinder 22 which nevertheless is sufficient to liftthe piston 23 and position the core box 25 and transfer chamber 14against the crosshead 11. The restricted flow of fluid pressure is suchthat it will allow a predetermined safe rate of lifting travel of thelift piston 23 and prevent the core box 25 from being slammed or bangedagainst the blow plate 38 of the sand transfer chamber. The main fluidpressure supply line 45 also communicates with the lift piston cylinderthrough a normally closed fluid pressure operated valve 96 which isbiased in the closed position by a spring 97 in the valve casing 98. Theend of the valve casing 98 opposite the spring 97 is in communicaf tionthrough ano-ther conduit 99 and fluid operated valve 160 with theconduit 56. The valve 100 is also normally biased in the closed positionby means of a spring 101 at one end of the casing 102 while the otherend of the casing 102 communicates with the branch conduit 103 which isconnected to the supply conduit 56.

When the lift piston 23 reaches the limit of its travel, after havingpositioned the core box and transfer chamber against the crosshead 11,the pressure builds up in the piston cylinder 22 and develops a unitpressure greater than the restricted pressure supplied by the con-y duit56 which is sufficient to overcome the spring bias on the valve andposition the Valve to establish communication between the conduit 56 andthe valve casing 98. The valve 96 is positioned under fluid pressurefrom conduit 99 to establish communication between the main fluidpressure supply line 45 and the lift piston cylinder 22, therebysupplying full fluid line pressure to the lift piston cylinder 22 tomaintain the core box and transfer chamber in secure clamped coreblowing relation' against themcro'sshead 11. y

The casing 106 of a fluid pressure operated differential' .exhaust valve107 is also connected at one end with the lift piston cylinder 22 and atthe other end with the Supply conduit 56. The'diferential valve 107normally maintains the exhaust vent S of the casing 106 closed lintilthe solenoid 60 is deenergized to isolate the supply conduit 56 from themain lluid pressure supply line 4S and remove the fluid pressure fromthe conduit 56 to allow the line pressure in the lift piston cylinder toexliaust and lower the core box from core blowing positionwfor readyremoval and replacement.

This lift piston arrangement may be readily coupled together with theother operative elements of the apparats into an overall automaticallyoperated system as shown in the. preferred embodiment of Fig. 1 and inthe circuit control diagram of Fig. 2. Thisis accomplished byyinserting" an additional contact switch 110 in series with the solenoid61 and con-trolling the switch so that the solenoid 61 is not .energizeduntil after the core box 25 'is positioned and clamped in core blowingrelation with the sand transfer chamber. ln the preferred embodiment ofFig. l, the switch contacts 110 are closed by another differential fluidpressure operated valve 112, having the large area side 113 of itscasing 114 coupled tothe conduit 105 and the small area side 115 coupleddirectly to the main fluid pressure supply line 45. The valve 112carries an operating lever 111 which is positioned with the valve 112 toclose contacts 110 mounted on the valve casing after the lift piston 23has reached its limit of travel and the pressure from the lift cylinder22 is suicient to overcome the balance of the line pressure at the otherend of the casing 114.

When the lift mechanism hereinbefore described is coupled together withthe control system and other operative elements of the apparatus, itenables a reduction in size of the lift piston due to the reduced orlimited opposing fluid pressure load attainable in the core box.Heretofore, it has been common practice to make the lift piston areaslightly larger than the supported surface of the largest core box. Thisled to cumbersome and uneconomical designs which can be overcome byutilizing the lift mechanism and apparatus described and which enablessafe operation with a lift piston of a size slightly greater than thesand filling opening of the transfer chamber.`

The circuit of Fig. 2 also illustrates additional refinements; one ofwhich is a normally open safety switch 116,

also in series with the solenoid 61, which has contacts 117 that may beclosed by the core box 25 or lift piston mechanism in core blowingposition.

Another refinement relates to the automatic starting and'stopping of theagitator 35 within the sand transfer chamber 14; As previously noted,the agita-tor 35 may be-driv'en by an electrical motor M. The energizingwindings 120 ofthe motor may be connected in parallel with the solenoids60 and 61 and in series with a switch 121 as shown in Fig. 2. The switcharm 121 is carried by'ajspring-biased movable valve 122 which ispositioned in its casing 123 by the increased pressure in the line 103when the core box and transfer chamber are positioned against the crosshead 11 to close the contacts 124 and energize the motor windings 120.

Thus; applicant has provided a sand blowing apparatus with a controlsystem which automatically determines the duration of the blowing cyclewhile maintaining certain safety precautions such as limiting themaximum determined safeV value' as well as limiting the rate of liftingtravel of the receptacle and transfer chamber to pre# vent damage duringpositioning. The system also ncludes other ancillary safety controls allofv which are automatically operated in proper sequenceY throughout theblowing cycle.

This apparatus and control system enables the automatic blowing of sandinto wooden or other frail sand receptacles without damage and enables areduction in' the size of the lift clamping pistons, thereby eliminatingcumbersome construction and providing a lighter weight and more`efficient blowing apparatus which operates automatically in an optimummanner.

T have shown and described what I consider to be the preferredembodiments of my invention along with similar modified forms andsuggestions, and it will be obvious to those skilled in the art thatother changes and modifications may be made without departing from thescope of my invention as described by the appended claims.

I claim:

1. A method of making sand molds comprising, blowing sand into amold-forming receptacle until the unit pressure therein reaches apredetermined amount which is less than the unit pressure of the source-of pressurev utilized for blowing the sand, then stopping the blowingof sand until the unit pressure in the receptacle is lowered, and thenrepeating the operation until the receptacle is lled withsand.

2. A method of blowing sand from a transfer chamber into a mold-formingreceptacle comprising, throttling the blowing pressure to limit theresultant unit pressure in the receptacle to an amount less than unitpressure of the blowing pressure source.

3. A method of blowing sand from a transfer chamber into a mold-formingreceptacle comprising, alternately admitting and releasing blowingpressure to and from the chamber until the receptacle is lled with sand,and controlling the alternate admission and release of blowing pressureto limit the unit pressure in the receptacle to an amount less than theunit pressure of the blowing pressure source.

4. A method of blowing sand from a transfer chamber into a mold-formingreceptacle comprising, admitting blowing pressure to the chamber until apredetermined safe amount of unit pressure is attained in thereceptacle, then stopping and releasing the blowing pressure from thereceptacle, and then repeating the operation until the receptacle isfilled with sand.

5. A method of making sand molds comprising, blowing the sand into amold-forming receptacle in pulsations until the receptacle is illed withsand and controlling the pulsations to limit the pressure in thereceptacle to a predetermined amount in the range between aboveatmospheric and below the blowing pressure of the sand.

References Cited in the le of this patent UNITED STATES PATENTS Re.23,817 Hill Apr. 27, 1954 1,769,081 Stevens July 1, 1930 1,889,163Vogel-Jorgensen Nov. 29, 1932 2,221,741 Vogel-Jorgensen Nov. 12, 19402,457,930 Smith Jan. 4, 1949 2,652,609 Sudia Sept. 22, 1953 2,682,692Kohl July 6, 1954 2,683,296 Drumm et al. July 13, 1954

